Controllable low noise gas discharge device



Aug. 5, 1958 w. M. WEBSTER, JR., ET AL 2,846,505

CONTROLLABLE Low NoIsE GAS DISCHARGE DEVICE i l 1/ /l/ 1 [11j/1 111// l l l l l l l J 11 A w. M. WEBSTER, JR.. ET A1. 2,846,695

CONTROLLABLE LOW NOISE GAS DISCHARGE DEVICE 2 Sheets-Sheet 2 :fg cof/rm afar/swf Min/ar Aug. 5, 1958 Filed Jan. 19, 1954 pas CNTRLLAELE LW 'NESE GAS DSCHARGE DEVCE William M. Webster, Jr., Edward 0. Johnson, and Louis Matter, Princeton, N. 5., assigner-s to Radio Corpora tion of America, a corporation of Delaware Application January 19, 1954, Serial No. 4ii4,981

1o claims. (ci. sis- 71) This application is a continuation in part of application Serial No. 341,946, led March 12, 1953, now abandoned.

This invention relates to discharge devices, or tubes, of the type comprising a sealed envelope containing an electrode system and an ionizable medium. In particular, this invention relates to improvmeents in gas discharge devices having an apertured electrode therein positioned between a cathode and other electrodes within the envelope.

In the gas discharge devices of presently known construction, when a potential dierenee that is sufficient to sustain a gas discharge is impressed between the various electrodes, conduction is accompanied by spurious variations of voltage and current which constitute noise components. Furthermore, if the power source which supports the discharge is connected across the electrodes through a resistor, it is observed that the potential difference between these electrodes experiences substantial oscillations or fluctuations that may be fairly regular in time or may approach in their randomness the characteristics of so-called random noise. In some cases, the variations in the potential difference between the electrodes are associated with characteristics of the external circuit and can, at times, be suppressed by suitable alterations or shielding in the external circuit. In other cases, they are a consequence of the internal processes in the ionizable medium and therefore cannot be materially reduced by modiiication of the external circuit. These latter variations are often of considerable amplitude; noise pulses of as much as 25 volts have often been observed.

it is because of the variations in potential difference between the electrodes that nearly all gas discharge -devices generate so much noise as to be useless in, or near, sensitive equipment unless extensive shielding and 1'ilter ing is done between the various components.

We have found that these large amplitude noise components never occur when the gas discharge device operates in a stable fashion. Generally, we have found that the operation of noiseless gas tubes may be attained when the voltage drop across the tube, or arc drop occurs entirely within a plane, located at some electrode. When this electrode is the anode the stable fashion is the anode glow mode of operation that is described in the papers entitled, Studies of Externally Heated Hot Cathode Arcs, Part I, Modes of the Discharge, RCA Review, September 1951, page 415, and Fart III The Anode Glow Mode, RCA Review, June 1952, page 163;

both of these articles were written by the present inventers. The anode glow mode may be attained by close spacing between an anode and cathode, i. e., in the range of 7 to 25 mils, at a gas pressure of less than 4 mm. of Hg. A disadvantage of the anode glow mode is that in most structures the current density that may be obtained from the device is fairly low and thereforethe applications of such devices are somewhat limited.

A further disadvantage of gas discharge devices prestent O i'atented Ang. 5, 3.953

ently available is that once current has started to liow through the device, this current cannot be interrupted by the application of a potential to a control electrode, usually a grid, as can be done in vacuum tubes. Because of this'limitation, the normal operation of gas discharge devices has been to remove the anode voltage and then wait for the device to deionize. This complicates the attendant circuitry and also restricts gas tubes to relatively low frequency applications.

It is therefore an object of this invention to provide a new and improved gas discharge device.

Another object of this invention is to substantially eliminate noise components and oscillations that take place in a gas discharge tube, by providing a gas discharge tube wherein the are drop occurs entirely within a plane located at an apertured electrode.

A still further object of this invention is to provide a new and improved noise free gas discharge device that is capable of conducting large magnitude currents.

A still further object of this invention is to provide a gas discharge device that is capable of cutting off the ilow of current between an anode and a cathode by the application of a small potential to a control electrode.

A still further object of this invention is to provide a new gas discharge device wherein the problem of deionization is eliminated.

Gas discharge devices utilizing our invention include at least a cathode, an apertured electrode and an anode. By providing a specilic correlation between (l) the type of ionizable medium, (2) the pressure of the ionizable medium, (3) the apertured electrode to cathode spacing, (4) the apertured electrode to anode spacing, and '(5) the size of the foramina in the apertured electrode, a noise free gas discharge device is constructed in accordance with our invention. These items are so correlated that the device operates in only one type of discharge, and the arc drop for the device occurs within the plane of the apertured electrode. When a device is constructed in accordance with this invention, it is noise free and current flow between the cathode and anode can be controlled by the application of small potentials to the control electrode. As will be explained hereinafter, continuous control of the current flow is possible. However, only initiation and interruption of the current flow is practical at this time. Therefore, the invention will be described as an on-o device. The above objects and other features and advantages of our invention will best be understood from the following description of the illustrated embodiments when read in connection with the accompanying drawings wherein like reference characters designate similar parts throughout the several views and in which:

Figure 1 is a graph of a ratio of the arc drop V across the tube to the ionization potential Vg versus a product of the pressure P of the ionizable medium times a normalization parameter, K,

Figure 2 is a schematic diagram of the potential distribution along the conduction path existing in a gas discharge device operating in accordance with our invention;

Figure 3 is a fragmentary view of an apertured electrode showing some of the principles that are to be considered in our invention; l

Figure 4 is a graph of the current output l for noise free operation, versus the pressure P of the ionizable medium times a normalization parameter, K;

Figure 5 is a longitudinal sectional view of a gas discharge device constructed in accordance with this invention;

Figure 6 is a transverse sectional view of a gas discharge device constructed in accordance with this invention;

Figure 7 is a graph of the characteristics of a de-V vice in accordance with this invention showing the cut off features of the device, and

Figure 8 is a graph of the characteristics of a device in accordance with this invention showing the continuously controllable features of the device. l

Referring now to Figure l for consideration of the parameters necessary to construct a gas tube in accordance with this invention that operates in'a stable, noise free, conditiorn'and one which is capable of being controlled by the application of potentials to the grid, there is plotted a ratio of the tube voltage drop V to the ionization potential Vi of any ionizable medium, versus the product of a normalization parameter K (to be explained hereinafter) times the pressure P of the ionizable medium used. When a gas tube operates on this curve, large currents can be conducted while the device is stable in operation, is noise free, and these large currents can be controlled by the application of potentials to the control electrode. As will be explained in connection with Figure 8, tubes constructed in accordance with this invention are capable of continuous grid control. However, at the present time it is practical to operate the devices only as anode or grid controlled on-o devices and the following description will accordingly be directed to the on-ol features.

We have found that the vast majority of the observed data may be placed along t'ne curve of Figure l if the proper value of the normalization parameter K is determined by the following equation:

where: W is a constant determined by the type of ionizable medium as will be hereinafter described; a is the average distance an electron will travel after leaving the apertured electrode before being collected, and in this case, is the apertured electrode to anode spacing, expressed in cm.; O is the optical transparency of the apertured electrode or when possible the effective transparency; and r is the cathode to apertured electrode spacing, expressed in cm.

The optical transparency is a ratio of the total area of the openings in the apertured electrode to the total area of the apertured electrode, while the effective transparency takes into consideration the sheath thickness as will be explained hereinafter.

The normalization parameter K is a parameter that has been determined to relate the various types of gas discharge devices so that a noise free tube is developed without the necessity of utilizing widely different calculations for the various types, or applications, of gas discharge devices.

The curve, as shown in Figure l, is a graphic representation of the noise free and the cut on regions. The curve is applicable to a great number of various structures that have been constructed and tested.V The cut o region extends from a pressure zero, i. e., vacuum tubes, to point F, but only the region A-F is useful for gas discharge devices. Therefore, the region A-F is considered as the cut off region, while only the region B-E is a noise free region. For the vast majority of uses it will be preferable to construct a tube which is noise free and also is capable of being cut off. Therefore, the smaller region, i. e., region iS-E, is preferably utilized, in this invention. in the region D-Ef the curve is extremely accurate. As the noise limit that occurs when V/V,=3 is approached some structures will operated at points slightly away from the curve. in other words, at the lower pressures the curve broadens for some structures. However, a structure can readily be corrected by varying the pressure slightly and the lower noise limit is then accurately defined.

It should be noted that noise will be present in the device unless the Ypressure of the gaseous atmosphere lies Vsize of the foramina in the apertured electrode.

i dbetween certain points. The noise that is encountered when V/V, is greater than 3, i.e point 3, has to do with the nature of the ionizing processes. While the details of the mechanisms producing noise at low pressures are not well understood, we have determined the minimum pressure that may be used in a noise free gas dis- Vcharge device constructed in accordance with our invention. This minimum pressure for noise free operation is expressed by the following relation.

Minimum pressure for noise free operation in mm. of Hg:

.014 Equation 2 P m where: W is a constant determined by the type of ionage distance an electron will travel after leaving the apertured electrode before being collected, and in this case is the apertured electrode to anode spacing, expressed in cm.; r is the cathode to apertured electrode spacing expressed in cm.; and O is the optical transparency or where possible, the effective transparency.

Equation 2 determines the minimum pressure for noise free operation. We have determined that the cut oit region extends beyond this minimum pressure to vacuum tube pressures. However, only pressures approaching point A on the graph of Figure l are useful as gas discharge devices. substantially follows the curve shown in Figure l. ln the A-B region of the curve the device will be noisy although it is capable of being cut oi Fora practical gas discharge device having'grid operated on-off" characteristics, an approximation of Equation 2 may be utilized to obtain the minimum useful pressue, i. e.,

(Equation 2a) .014

P zw: W\/ar0 with the dimensions for Equation 2a being the same as in Equation 2. Tubes constructed with pressures below the pressure obtained by Equation 2a will have an on-oif characteristic, but known vacuum tubes will have the same power ratings thus the features of gas tube construction would not be practical at these lower pressures.

The width of the region D-E (this is the noise free region for most eicient operation of a thyratron or a rectilier type of gas discharge device) depends upon the For a large size of foramina in the apertured electrode, the foramina still being less than the mean free path of the plasma particles, the region DE is small. For smaller foramina the region D-E is much greater, and has been found to include a pressure range as great as 10 to l, i. e., the pressure at which point E occurs is 10 times the pressure at which point D occurs. It has been found that a mesh having about 200 apertures'per square inch gives such a small region D-E as to be comparatively'useless for noise free rectifier or thyratronuse. Due to manufacturing problems, the other extreme, i.V e., a very ne mesh is impractical also. It has been found that mesh sizes between 400 and 3,000 apertures per square inch will generally give the best results. This invention is not to be limited to these values but they are given merely as practical limits with the actual limit for noise free operation being foramina that are no greater in maximum dimension than the length of a mean free path of the plasma particles.

Analysis shows, and experience confirms, that Where the largest dimension of the foramina exceeds the mean free path of the plasma particles, the discharge will exhibit large amplitude noise. Since mean free path is inversely related to pressure, this criterion places an upper limit on pressure for a given sizeof foramina or an upper limiton foramina dimension for a given gas pres- The low pressure region, i. e., A-B,

sure). We have found that the best single criterion to de# line an upper pressure limit for noise free operation, i. e., point E, is that the largest dimension of the foramina be no greater than the length of a mean lfree path of an electron of ionizing energy in the medium with which the tube is lled. The region wherein a gas discharge may be cut ott extends slightly beyond the point 13, i. e., to point E In other words, for cut off operation, the lforamina in the apertured electrode may be slightly greater in size than the length of a mean free path of the plasma particles.

A constant, Gj may be defined toV relate mean free path to pressure for various gases, and the upper pressure limit may then be expressed mathematically in terms, of G, and the foramina dimensions by the relation:

Square foramina- E nation 3 P q 1/2(d2R where: G is the mean free path in cm. of an electron of ionizing energy at a pressure of 1 mm. of Hg; d is the distance between wires measured from center to Center and expressed in cm.; and R is the radius of the wire expressed in cm.

Equation 4 G P S where: S is the radius of the foramina.

In Equations 3 and 4, G is determined by the type of ionizable medium and may be found by referring to any of the standard gaseous Aconductor handbooks or by If it is desired to operate the device in the region E to R an approximation of Equations 3 and 4 may be utilized. Thus P max. @mi

(Equation 3a) for square foramina, and

G P max. -2S

(Equation 4a) for round foramina.

lt should be understood that both upper and lower pressure limits should be computed for any structure since it is conceivable that some arrangement of electrode spacings would result in the lower limit defining a higher pressure than the upper limit. In such a case, there would be no noise free region although there may possibly be a cut off region.

The type of ionizable medium determines the gas constant W. It is known that W includes the diiierential ionization coeflicient, and a ratio of the ion mean free path length to an electron mean free path length at the energy level involved. Due to the diiculty of determining these items, an experimental method of determining W is as follows. Construct a discharge device cornprising a cathode, an apertured electrode having apertures that are smaller than the mean free path length, and an anode, having spacings as follows: between cathode and apertured electrode a spacing within the range 0.1 cm. t'o l.5 cm.; and between apertured electrode and anode a spacing within the range of 0.1 cm. to 3 cm. When the asaaeos device is constructed it should then be lilled with an ionizable medium and the pressure varied until the arc voltage drop V across the device has a value for which .K P, in the following equation is known, then tinally the values of the distances should be substituted into the following equation:

where: W is the gas constant; KXP is a constant having a `value greater than 0.014 and may be determined from Figure l; P is the pressure of said ionizable medium, expressed in mm. of Hg; O is the optical transparency of the apertured electrode or where possible the effective transparency; a is the average distance an electron will travel after leaving the apertured electrode before being collected by an electrode, and in this case is tre apertured electrode to anode spacing expressed in cm.; and r is the cathode to apertured electrode spacing expressed in cm.

One point on the graph of Figure l where the constant K P is known is where the arc drop V of the device is equal to twice the ionization potential V1 of the ionizable medium. The value of K P at this point is ..02 and occurs at point C on the graph of Figure 1. Thus, Equation 5 may be calculated for this point. Various other specific points, to determine the value of K P, may be obtained from the graph shown in Figure l. Since the arc drop V may be measured, and the ionization potential V is known for the various ionizable mediums, the value of K P7 may be determined from the graph shown in Figure 1. Since the pressure P may be measured, i, e., the pressure at which lf/V will be at a point on the graph, all of the elements in Equation 5 are known or may be measured.

lf it is desired, one of the following ionizable mediums may be used for which the gas constant W has been determined and is as follows:

Table No. 2

Gas W Helium is 1 Neon is 18 Argon 1s 13 Xenon 1s 25 The point C on the graph of Figure 1 occurs at the point where the product of the normalization parameter Y times the pressure P equals .02, point D occurs at .07, and point E occurs in the neighborhood of 0.2. For the above mentioned values, the pressure is expressed in millimeters of mercury. The value for was determined partially by analysis and largely by empirical means and r has been tested over the range of 0.1 cm. to 1.5 cm.; "a over the range .1 to 3 cm. and; O over the range .1 to 1.

Referring now to Figure 2, there is shown schematically a cathode l0, which may be of the conventional thermiom'c type of oxide coated cathode, an apertured electrode T12, and an anode i4. Tae distribution of space potential between these electrodes is shown as a line 16. The space potential diagram shown is the type of distribution attained for a noise free device constructed in accordance with this invention. lt should be noted that the potential drop between cathode 10, and anode 14 (commonly referred to as the arc drop), is almost entirely within a smooth plane located at the apertured electrode 12. When operating properly, one sees a glow uniformly illing the regio-n between the apertured electrode 12 and the anode 1d. The region between cathode lll and apertured electrode 1.2 should be darle Because the arc drop is made to occur in the plane of the apertured electrode l2, the device may be operated at a much greater current density than is possible when utilizing the anode glow mode referred to above. While the reasons for this are not completely understood at the present time, it is Va fact. Having the arc drop occur within the plane of apertured electrode 12, also permits substantial control over the arc drop which is of considerable value, i. e., a more negative potential on apertured electrode l2 increases the sheath thickness, decreases the eiiective transparency and thus increases the arc drop V. As an example, when utilizing a discharge device of the type claimed in a copending application of E. O. Johnson, tiled September 20, 1950, Serial Number 185,745, now abandoned and co1 inuation application Serial Number 583,443, filed May 8, 1956, in its stead and assigned to the same assignee as the present invention, the optimum arc voltage drop V in the auxiliary discharge path is approximately twice the ionization potential Vi of the gas. This value gives most ecient over-all operation. VOn the other hand, the arc drop V for rectifier, or thyratron types of tubes is usually as low as possible to give high eliciency- A further feature of placing the arc drop within the apertured electrode plane is that the apertured electrode 12 may be used to render a thyratron type of tube free or high amplitude Vnoise while simultaneously functioning as a thyratron grid. A still further feature of placing the arc drop within the plane of the apertured electrode is that now a negative potential may be applied to the apertured electrode l2 which causes the adjacent sheaths to overlap thus completely cutting oit the flow of current between the cathode 1t? and the anode 16. The closure of the apertures in electrode l2 by means of the expanding sheaths has been observed to take place in less than a microsecond which eliminates the deionization time problems from gas discharge devices.

Referring now to Figure 3, for a consideration of the items necessary to attain the space potential distribution shown in Figure 2, there is shown an enlarged fragmentary section of a screen, or mesh, type ofapertured electrode l2 while in a discharge device-that is conducting. Proximate of the apertured electrode 12 and on both sides thereof is a region of plasma 15 and 15a. A plasma is a concentrated region of equal densities of free electrons and positive ions which is consequently nearly equipotential. The openings, or forarnina, between adjacent wires 13 are smaller than the length of the Vmean free path of the plasma particles for noise free and cut off operation.y For cut o operation alone the openings between adjacent wires 1 3 .may be slightly greater than the mean free path of the plasma particles, i. e., region E-F of Figure l. Due to the fact that the openings are, at most, only slightly larger than the mean free path of the plasma particles, what might have been a continuous plasma, is divided into two separate plasmas, i. e., the anode plasma l5, and the cathode plasma 15a. Plasma region i lies between the apertured electrode 12 and the anode 14 of the discharge device, while plasma'region 15a lies between the cathode 10 and apertured electrode 12. Region has a space potential nearly equal to that of the anode electrode 14 while the potential of plasma region 15a is slightly negative with respect to the surface of cathode 10.

Between the two regions of plasma is a region of high electric iield lb which extends through the foramina of the electrode l2. The positive ions which diffuse through plasma region l5, to the edge of region 15b are accelerated by the field in the direction of region IlSa. Likewise, electrons which diffuse to the edge of region 15a are accelerated in the direction of region 15. Some ions and electrons are intercepted by electrode 12, the remainder passing completely through the region 15b. The fraction of the electrons and ions intercepted is not only related to the optical transparency of the apertured electrode 12 but is more directly determined by the sheaths which surround the individual members (e. g., wires) which constitute the electrode i2. That is to say, the etfective size of the forarnina for penetration of ions and electrons during conduction is less than the actual size due to the sheaths. Such sheaths are regions containing only positive ions or negative electrons en route to the electrode. A more detailed and complete explanation of the plasma, and the sheaths, may be `found in an article entiled, Rapid determination of gas discharge constants from probe data by Malter and Webster appearing in the RCA Review, volume 12, Number 2, June-1951.

Normally, the edective transparency of an apertured electrode is somewhat less than the optical transparency due to the sheath thickness. Thus the optical transparency for a mesh type of apertured electrode is equal to:

:(dwzan Equation 6 where: O is the optical transparency; d is the distance between wires measured from center to center and expressed in cm.; and R is the radius of an individual wire.

Round foramina- 2 Equation 7 0:(-in-lwhere: O is the optical transparency; n is the number of foramina per square cm.; and S is the radius of a oramen in cm.

Thus, with the wire spacing df and the wire-radius R, the width of each foramen is t-2R as shown in Figure 3.

lf the calculation of the transparency of anapertured electrode is to be completely accurate, the eective transparency is calculated, i. e., the sheath thickness should also be taken into account so that the width of each wire 13 to the edge of its surrounding sheathv 17 and expressed in cm. f

The sheath thickness N varies with the voltage that is applied to thel apertured electrode-12, as well as with certain other factors. However, in some instances it is possible to make a reasonable calculation of the sheath thickness N A method of calculating, to a rst approximation, the sheath thickness N is found in the aboveidentified article by Malter and Webster, Volume 12, Number 2 of the l une 1951, RCA Review. An example of when this may be done is when the voltage applied to the apertured electrode 12 remains constant. When the sheath thickness N can be so computed the eiective transparency O of the apertured electrode 12 is used which is:

Equation 8 Equation 9 where: O isrthe effective transparency; n is the number of foramina per square cm.; S is the radius of foramina and N is the sheath thickness expressed in cm.

Values of O for common sizes of mesh type apertured electrodes have been obtained from the wire spacing, Wire diameter, and an estimate of sheath thickness are as follows:

Table No. 3

It should be understood that other expressions for the dimensions, i. e., inches etc., may be used for the above formulas as long as all dimensions are expressed in the same scale.

The sheath thickness N is a second order consideration so that it may often be neglected if a variable voltage is to be applied to the electrode 12 or if some other feature of the device is present that will vary the vsheath thickness N and thus make calculations extremely diicult. When the sheath thickness N is neglected, the design structure as given by this invention, Will usually be sufficiently exact.

There is, in addition to the variation of arc drop with pressure and geometry, a variation in the maximum noisefree current which can be drawn thru the tube With the same parameters. The general nature of the variation of this current with pressure is illustrated by the curve of Figure 4. The data obtained for a variety of geometrically diierent tubes are too diversied to be placed in any simple empirical equation. However, assuming a concentric arrangement of electrodes, generally the maximum current decreases if the spacing a becomes too small (smaller than the mean free path of an electron with the velocity associated With the arc drop), and normally increases as r is increased simply because more'apertured electrode area exists so that more total current is carried Without having to increase the current density thru the apertured electrode. Since cz-i-r deiines the anode radius, there must be a compromise to effect maximum noise-free current where this end is essential. in experiments limited to tubes of anode diameter in the neighborhood of 2 cm., it was found that the optimum compromise values were in the region of al cm. These dimensions were not found to be critical and other values will pass suicent current in the noise free mode for most applications. Thus, it is not the intent of the present invention to be limited to these above dimensions as given.

A rectifier tube 2i constructed in accordance with this invention is shown in Figure 5. In this figure a thermionic cathode 2li is surrounded by an apertured electrode 22. Surrounding the apertured electrode 22 is an anode 24. Envelope 25 has the conventional stem 29 that supports the electrode structure in the usual manner. The electrodes are supported by end insulating members 26 and 27 and are held in position by support rodsY 23 in the conventional manner. For rectifier operation it is usually desirable to have the potential drop, or arc drop, across the tube at a minimum; therefore, the region D to E as shown in Figure 1 is the most desirable for this type of operation. It is for this reason that the product of the normalization parameter l times the pressure P should be greater than .O7 for rectifier type of operation.'

The structure shown in Figure may be used as a noise free gas discharge rectier by allowing apertured electrode 22 to float. If desired, the structure may be used as a noise free thyratrc-n with grid cut-off action by using apertured electrode 22 as the thyratron control electrode.

In order to give an example of how the invention is to be utilized to obtain a noise free thyratron or rectiiier, the following dimensions have been assigned to the struc- 10 ture shown in Figure 5. These dimensions are not given with. the intent of limiting the invention but' are given merely as an example of how the invention is applied to a structure.

The rst element to consider is Where on the curve shown in Figure 1 does efficient operation of the thyratron or rectier devices occur. As has been stated, the region D-E is most ecient for this type of operation. Since P is .07 at point D (from the graph of Figure 1), K P may vary above this point and the operation will be eicient.

In the specic structure a standard size thermionic cathode 20 is utilized having a diameter of .100 inch. Surrounding the cathode 2@ is an apertured electrode 22 having a diameter of 1.0 cm. The apertured electrode 22 is constructed of 40 x 40 wires per square inch, each wire having a diameter of .010 inch. Coaxially spaced around the apertured electrode 22 is a tubular anode 24 having an inside diameter of 2 cm. The entire structure is l inch in height; however, this is immaterial. Assume that it desired to utilize a filling of argon.

Thus for noise free operation, the minimum pressure for the above structure is (from Equation 2') memes mm,

The maximum pressure for the above structure is (from Equation 3):

P max.

.014C T* :.26 mm. V2 (nas-.01mm

Where: G is equal to .014 from Table 1; d is equal to .025 inch; and R is equal to .005 inch.

Thus the pressure range of the above described tube for noise free operation is 16.5 microns to 260 microns. As was explained above, the cut ott range of pressures is slightly larger than the noise free range of pressures. However, a great number of factors must be considered to determine this extended pressure range and since, for the vast majority of uses, it is desirable to have both noise free and cut oit type operation, accurate determination of this extended range is not deemed necessary. An assumption of the extended range for the specific example of structure is a gas pressure Within the range of 10 microns to 275 microns. ln order to determine what pressure from this range should be used in the tube, an approximation may be made of the D to E region of Figure 1. Thus a value near the upper pressure limitwould be desired and an assumption of pressure of say 25 O microns could be used.

ln order to more accurately determine the region of desired pressures for a rectier or thyratron type of tube, Equation 1 may be used to determine Where point D occurs, thus (from Equation 1):

where: K P is equal to .07; W is equal to 13 from Table 2; "a is equal to 50 cm.; O is equal to .25

i l. l from Table 3; and "1" is .37 cm., the D to E region is varied over a pressure range of 82 microns to 260 microns and the assumption of 250 microns is well within this range and may be used in this structure.

Referring now to Figure 6, there is shown a cross sectional diagram of one structure for the device described and claimed in the above identied copending applicav tion. This device will also be assigned specic values to demonstrate the use of the invention for this type of discharge device. The device comprises an auxiliary thermionic cathode 34 having a diameter of 50 mils. Surrounding the auxiliary cathode 34 is an apertured elec* trode 36 having a diameter of 1.5 cm. The apertured electrode 36 is a 50 x 50 mesh electrode utilizing wires .010 inch in diameter. Spaced every 90 around the auxiliary cathode are four main cathodes 3S each having a diameter of 1A inch. Surrounding this entire structure is a' control electrode 40 and an anode 42. The control electrode wires 40 are .010 inch wires spaced 2 mm. apart, and spaced from anode 42 a distance of 1 mm. The gas lling is helium. The electrodes are supported in a conventional manner (not shown) within envelope 43. In operation, a discharge occurs 'between the auxiliary cathode and the main cathodes. It is desirable that this discharge not generate noise.

In the device shown in Figure 6, the dimension r is .7 cm. The distance a is the average distance an electron will travel after leaving apertured electrode 36 before being collected by an electrode. In order to determine the effective value of "a, a calculation must be made considering the various distances between the apertured electrode 36 and the anode, main cathodes, and the control electrode as well as the percent of the electrons in the auxiliary discharge intercepted by each of these electrodes. The spacings for the particular structure shown are as follows:

a1=1.75 cm.=distance from apertured electrode 36 to a cathode 38;

a2=3-60 crn.=distance from apertured electrode 36 to control electrode 40;

3:31.80 cm.=distance from apertured electrode 36 to anode 42.

where: W is 1 from Table 2; cf is 3.2; r is .7; and O is .10 from Table 3.

` The maximum pressure for noise free operation is (from Equation 3):

G \/(d-2R)cm.

P max.

where: G from Table Vl is equal to .12; d is equal to .02 inch; and R is equal to .005 inch.

Thus the noise free range of pressures for the above described structure is Within the range of .112 mm. of Hg to 3.4 mm. of Hg. As has been pointed out, the type of tube disclosed in the above identiiied copending ap-k plication operates more eiiiciently at point C Thus the pressure at point C is:

160 mm. Of'Hg P (at C):

operation was obtained. The probable error of pressure measuring equipment exceeds the difference between .160 mm. and .180 mm. of Hg. n

Referring now to Figure 7, there is shown a set of characteristics for gas discharge devices constructed in accordance with this invention. The characteristics are for a three electrode type of device as shown and described in connection with device 21 of Figure 5. From these characteristics, it is shown that in order to initiate a discharge which the control electrode 22 is holding off, i. e., at point X, the bias on the control electrode 22 must be made more positive until the ring characteristic line is crossed, i. e., point Y. This is conventional thyratron operation. Now, when it is desired to cut oi the discharge, the control electrode bias is made more negative until the particular operational line (determined by the external circuit) is crossed, i. e., to point Z on the milliampere curve. When the operational line presently being utilized is crossed, the positive ion sheaths overlap each other so that the gas discharge is cut oi. The device 21 remains cut off until the control electrode 22 is made more positive to cross the tiring characteristic as was described.

In a typical developmental tube of a 100 milliampere size, a control electrode bias of `20 volts affects the interruption of 100 milliamperes of anode current when the supply voltage is 200 volts.

p Referring now to Figure 8, there is shown a set of' characteristics for a gas discharge device 21 while being operated as a continuously controllable gas discharge device. As in conventional practice, a particular load line, which is determined by the external circuit, is utilized. The device 21 cuts olf at the grid potential Ec -which is tangent to the load line. Within the range of grid biases between cut olf bias and zero grid bias, the device may be utilized as an amplifier. In other words, a change in grid bias Ec results in a change in anode current resulting in amplification. The tube drop Vb may also be varied to vary the anode current.

The reason that the present invention has been described as an on-of device is that when the device is utilized as an amplifier, as disclosed in Figure S, the tube life is extremely short due to sputtering of the control electrodeV material. In fact, the tube from which Figure 8 was compiled was destroyed in a matter of a few hours. Thus, it is not practical to operate the device as ian amplier until it has been determined what material may be utilized as a grid to provide a long tube life. However, when the grid` bias is rapidly switched through the region between zero grid bias and cut-olf bias, tubes of this type have been in operation for over 2,000 hours.

- While we have indicated the preferred embodiments of our invention of which we are now aware and have also indicated several specific applications for which our invention may be employed, it will be apparent that our invention is by no means limited to the exact forms illustrated or the uses indicated, but that many variations may be made in the particular structure used and the purposes for which it is employed without departing from the scope of our invention as set forth in the appended claims.

We claim:

1. A substantially noise free gas discharge device hav-k ing an envelope containing an ionizable medium at a lt was iilled with helium to a` pressure of .18 mm. of Hg at'which pressure optimum,

gare-,eos

given pressure, a thermionic cathode, a fo'raminous" electrede, a main cathode, a control electrode and an anode arranged in that order within said envelope, the maximum dimension of the foramina in said foraminous electrode being less than the mean free path of the electrons of said ionizable medium, and said electrode geometry being such that the ratio of the arc voltage drop to the ionization potential at said pressure is less than 3.

2. A substantially noise free gas discharge device, comprising a sealed envelope, said envelope having an ionizable medium therein, a thermionic cathode electrode, an apertured electrode and an anode electrode arranged in that order, the pressure of said ionizable medium being determined in accordance with the following equation:

where: K P is a constant depending upon the type of operation desired; W is a gas constant depending upon the type of ionizable medium; a is the average distance in cm. an electron will travel from said apertured electrode before being collected by an electrode; O is the eective transparency of said apertured electrode; and r is the spacing in cm. between said apertured electrode and said cathode; the electrode relationship between said electrodes being such that said pressure occurs within the range determined by the following relationship:

where: P is a range of pressures expressed in mm. of Hg; W is a gas constant depending upon the type of ionizable medium used in said envelope; a is the average distance in cm. an electron will travel from said apertured electrode before being collected by an electrode; r is the spacing in cm. between said apertured electrode and said cathode; O is the effective transparency of said apertured electrode; G is the mean free path in cm. of an electron of ionizing energy at apressure of l mm. of Hg; d is the distance in cm. between adjacent solid portions of said apertured electrode measured from center to center; and R\is the radius in cm. of said solid portions of said apertured electrode.

3. A substantially noise free gas discharge device, comprising a sealed envelope, an ionizable medium in said envelope, an auxiliary thermionic cathode electrode, an apertured electrode, a main thermionic cathode, a control electrode, and an anode arranged in that order in said envelope, the pressure of said ionizable medium being within the range determined by the following expression:

.014 G Won/E (tz-2m@ where: P is a range of pressures of said medium expressed in mm. of Hg; W is a gas constant depending upon the type of ionizable medium used in said device; a is the average distance in cm. an electron will travel before being collected by an electrode after leaving said apertured electrode; r is the spacing in cm. between said auxiliary cathode and said apertured electrode; O is the eiective transparency of said apertured electrode; G is the mean free path in cm. of an electron of ionizing energy at a presure of l mm. of Hg; d is the distance in cm. between adjacent solid portions of said apertured electrode measured from center to center; and R is the radius in cm. of the solid portions of said apertured electrode.

4. A substantially noise free gas discharge device having an envelope containing an ionizable medium, a cathode, a foraminous electrode and an anode arranged in 14 that order within said envelope, wherein the product of a constant K and the pressure of the ionizable medium within the device is not less than .014 and not greater than .2 and wherein:

where: W is a gas constant determined by the type of ionizable medium used in said envelope; a is the average distance in cm. an electronwill travel from said apertured electrode before being collected by an electrode; O is the effective transparency of said apertured elec-` trode; and r is the spacing in cm. between said apertured electrode and said cathode.

5. A gas discharge device of the type wherein a control electrode is capable of initiating and interrupting a gas discharge between a cathode and an anode comprising, a sealed envelope containing an ionizable medium at a given pressure, a cathode,- an apertured control electrode and an anode' supported in that order within said envelope, the maximum dimension of the apertures of said apertured electrode being not substantially greater than the mean free path of electrons of said ionizable medium, and the geometry of said device being such that the ratio of the arc vvoltage drop through said device to the said ionization potential at said pressure is less than 4.

6. A substantially noise free gas discharge device of the type having a control electrode capable of initiating and extinguishing a gas discharge therein comprising, a sealed envelope, an ionizable medium in saidpenvelope at a given pressure, a cathode, an apertured control electrode and an anode arranged in that order within said envelope, the maximum dimension of the apertures of said apertured electrode being no greater than the mean free path of electrons of said ionizable medium, and the geometry of said device being such that the ratio of the arc voltage drop through said device to the ionization potential at said pressure is less than 3.

7. A substantially noise free gas discharge device capable of having a gas discharge therein cut oit, comprising a sealed envelope, and having at least a thermionic cathode, an apertured electrode and an anode arranged in that order in said envelope, an ionizable medium within said envelope, and said ionizable medium being at a pressure within the range determined by the following relationship:

where: P min. is the minimum pressure of said medium expressed in mm. of Hg; P max is the maximum pressure of said medium expressed in mm. of Hg; W is a gas constant depending upon the type of ionizable medium used in said envelope; a is the average distance in cm. an electron will travel from said apertured electrode before being collected by an electrode; r is the spacing in cm. between said apertured electrode and said cathode; O is the optical transparency of said apertured electrode; G is the mean free path in cm. of an electron of ionizing energy at a pressure of l mm. of Hg; d is the distance in cm. between adjacent solid portions of said apertured electrode measured from center to center; and R is the radius in cm. of said solid portions of said apertured electrode.

8. A substantially noise free gas discharge device having an envelope containing an ionizable medium, a cathode, a foraminous electrode and an anode arranged in operative relationship within said envelope, the size of the foramina being less than the mean free path of the electrons of said ionizable medium, and said ionizable medium having a minimum pressure determined bythe following relationship:

where: P min. is the minimum Vpressure of said medium expressed in mm. of Hg; W is a gas constant depending upon the type of ionizable medium; a is the average distance in cm. an electron will travel from said apertured electrode before being collected by an electrode; "r is the spacing in cm. between rsaid apertnred electrode and said cathode; and O is the optical transparency of said apertured electrode.

9. A substantially noise free gas discharge device of the type capable of having a discharge cut off bythe -application of a potential to an apertured electrode comprising, a sealed envelope, at least `a thermionic cathode, an apertured electrode and an anode arranged in that order in said envelope, an ionizable medium within-,said envelope, said ionizable lmedium having Ia pressure within the range determined by the following relationship:

16 where: P min. is the minimum pressure of said medium expressed in mm. of Hg; =P max. is the maximum pressure 0f said medium'expressed in mm. of Hg.; W is a gas constant determined in accordance with Table 2 of the specification; a is the average distance in cm. an electron will travel from `said apertured electrode before being collected by an electrode; r is the spacing in cm. between said apertured electrode' and said cathode; O is the optical transparency determined iny accordance with Table3 of the. speciiication; G is the mean free path in cm. of an electron of ionizing energy at a pressure of 1 mm. of Hg; d is the distance in cm. between adjacent solidrportions of said apertured electrode measured from center to center; and R is the radius in cm. of said solid portions of said apertured electrode.

l0. A gas discharge device as in claim 6, wherein said ratio is greater than unity.

References Cited in the le of this patent UNTTED STATES PATENTS 

